Study On Mechanism Of Modifying Precipitates And Property Of AZ Magnesium Alloy By Pre-twinning Deformation

AZ series alloy with relatively high Al content is one of the most widely used magnesium alloys due to its high strength and low cost.During ageing treatment,there are two kinds of precipitates:discontinuous and continuous precipitation.Discontinuous precipitates(DP)are very coarse,which may act as crack initiation and deteriorate the strength and ductility.Compared to aluminum alloys,even continuous precipitates(CP)are larger in size and sparser in distribution.Therefore optinizing the morphology,size,distribution and orientation relationship of precipitates with the matrix can significantly affect the age hardening performance of Mg alloys.AZ80 rolling plate and AZ91extruded rodare used as model matirials in this work.Using EBSD,TEM and SEM,the influences of twinning deformation and ageing treatment on the shape,size,distribution and orientation of precipitates were systematically studied.In particular,the effects of different twinning deformation on the characterristics of precipitates were analyzed.Ageing temperature can largely affect the distribution of precipitates in AZ80magnesium alloy.Under optimized aging temperature(180?C),strong hardening effect can be obtained.But there were still a lot of discontinuous precipitates.By applying10%compressive strain along TD,the microstructure was completely twinned.Complete twinning was introduced by compressing samples to a strain of 10%along TD.Then,the coarse lamellar-shape discontinuous precipitates can be completely inhibited during the subsequent ageing treatment.This was attributed to that a large number of dislocations provided more nucleation positions for continuous precipitates.Compared with SA36h sample under direct ageing,the yield strength and peak strength of SCA36h sample were increased by 81 MPa and 62 MPa respectively for tension.They were by 80 MPa and 48 MPa respectively for compression.Moreover,the yield asymmetry was improved with the CYS/TYS ratio increasing from 0.69 to 0.78.Strain-path change compression was applied to generate multiple twins.Compared with samples under traditional direct ageing treatment,the mechanical properties of TD5RD4 samples(especially the peak strength and elongation)had been significantly improved for both compression and tension along RD,TD and TDRD45(i.e.,the diagonal direction).Firstly,the strain path changed compression not only caused grain refinement but also weakened texure.Secondly,the introduction of a large number of twins and twin boundaries can promote continuous precipitation,and hinder the movement of twin boundaries.Finally,discontinuous precipitation can also be completely inhibited.Compared to direct peak-aged alloy,the inprovements of tensile yield strength,peak strength and elongation for the TD5RD4 sample were 40 MPa,171MPa and 12.2%,respectively.The effect of detwinning deformation on the microstructure and properties of AZ80rolled plate was studied.1)It was found that the compression strain(5%or 8%)along TD and ND can cause complete detwinning of the sample,and the texure was restored to the original state.The precipitates became finer,dencer and more uniform during ageing treatment.With increase of deformation,discontinuous precipitates disappeared gradualy.The strength and anisotropy were significantly improved.2)For samples subjected to compression and then ageing treatments,orientation relationship of most precipitates with the matrix was the typical Burgers OR(with precipitates parallel to the basal plane).Some of these samples were compressed by 8%again along ND.The orientation relationship of precipitates in these samples are(01-10)_?//(011)_?,[-2110]_?//[11-1]_?(with precipitates parallel to the prismatic plane).3)Mechanical tests and calculations based on Orowan theory indicated that the precipitates with favored OR(i.e.,parallel to the prismatic plane)are effective in Orowan strengthening.4)A new OR was observed for continuous precipitation with the matrix in TD8-A36h sample:(0001)_?//(1-13)_?,[10-10]_?//[110]_?.Finally,AZ91 Mg alloy rods were used to investigate the effects of torsion deformation on the microstructure evolution and ageing hardening behavior.Extruded AZ91 rod has a uniform microstructure with typical fiber texture.After torsion,the area fraction of{10-12}twins gradually increases from the center to the edge in the cross-section of the rod,and the basal plane tends to rotate forward the ED.The precipitation behavior of the regions without twinning is dependent on the magnitude of deformation,and continuous precipitation was promoted by providing a large number of dislocations.Therefore,gradient distribution of precipitates was present with the gradient distribution of twins and dislocations in ST sample.This causes increased yield strength,reduced and even eliminated yield asymmetry.